Where SCR catalysts are employed to limit NO.sub.x emissions from diesel engines, one has to deal with either the dangers of ammonia or a risk of fouling the catalysts under most conditions. In this regard, see R. J. Hulterman: A Selective Catalytic Reduction Of NO.sub.x from Diesel Engines Using Injection Of Urea; Ph.D. thesis, September 1995. Hulterman describes a number of technical challenges including clogging of atomizers, decomposition problems and system dynamics.
The limited attempts to use urea SCR for diesel engines have required the use of large pyrolization chambers or other devices following the point of urea introduction into the exhaust, as disclosed in U.S. Pat. No. 5,431,893, to Hug, et al. Equipment of this type highlights the known problems with urea.
Urea takes time to break down in hot exhaust gases and may cause nozzle plugging. To protect an SCR catalyst from fouling, Hug, et al., propose bulky equipment. In addition, this disclosure highlights the necessity of maintaining the urea solution at a temperature below 100.degree. C. to prevent hydrolysis. They propose the use of moderate urea pressures when feeding the urea and find it necessary to have alternative means to introduce high-pressure air into the feed line when it becomes plugged. The nozzles employed by Hug, et al., are apparently capable of producing moderately-fine sprays, the dispersion of which is aided by auxiliary air, but the droplets are still so large that a large pyrolization channel is required. Moreover, they employ dilute solutions that require significant heating to simply evaporate the water. See also, WO 97/01387.
In European Patent Specification 487,886 A1, urea SCR is said to be improved by obtaining quantitative hydrolysis of urea outside the supply lines. The disclosure calls for spraying fine droplets of aqueous urea onto a specially-formulated catalytic surface. The objective of the disclosure is to avoid pyrolysis and its potential for solid deposition, while achieving hydrolysis of urea to ammonia and carbon dioxide.
In European Patent Specification 615,777 A1, there is described an apparatus that feeds solid urea into a channel containing exhaust gases, which are said to be hydrolyzed in the presence of a catalyst. For successful operation the disclosure indicates that it is necessary to employ a hydrolysis catalyst, compressed air for dispersion of fine solids, means for grinding the urea into fine solids and a coating to prevent urea prills form sticking together. The disclosure notes that if the inside of the catalyzer and the nozzle tip only were coated with the catalyst, corrosion and deposition occurred. Despite achieving the goal of removing water from the process, the specification introduces solid urea into the gas stream--possibly depositing urea on the SCR catalyst.
Each of the points of caution by these references about the difficulties of using urea with SCR systems, especially for mobile sources, illustrates the trouble and lack of practical success the art has had and continues to have.
The art is awaiting the development of a process and apparatus that would permit the use of urea or other reagent in an SCR process simply, reliably, economically and safely.